Pipelines that are used to transport products such as petroleum, gas or other fluids can become blocked or inefficient through the build up of deposits on the pipe walls. The deposits can be foreign material, detritus, or natural waste products such as, for example, paraffin, calcium, wax, hydrates, scaling, naftenat and asphaltenes. In order to remove these undesired deposits pipes and pipelines in general require cleaning, testing or gauging. A well known technique to obtain such removal is to use a so-called “pig” (Pipe Inspection Gauge). The pig is designed to fit closely within the pipe and is caused to travel along the pipe by admitting fluid under pressure behind the pig. During the travel the deposits are removed from the inner surface by scraping or brushing, or simply by pushing the deposits ahead of it as it travels to a point where it can be removed along with the released deposits. Such mono-directional pigs, which are transported along with the fluid flow, may become stuck when it encounters large amounts of pipe wall deposits, and thus form a permanent plug in the pipeline. Examples of traditional mono-directional pigs may be found in GB 2,141,201 A or FR 2,630,934 A1. Very recently bi-directional pigs have been demonstrated suitable for removing deposits within subsea hydrocarbon production flowlines, see WO 2012/093079 A2.
Removal of deposits is particularly important in the oil and gas industry. Severe problems often occur when hydrocarbon fluids are transported in long subsea pipelines at large depths and in cold waters. Such problems may include the formation of obstructions in the pipeline, in the form of hydrates or other deposits such as ice, wax and debris (e.g. asphaltenes, sand). The initially warm well fluid is cooled down by cold seawater, thereby inducing condensation, precipitation and hydrate and wax formation/crystallization. Below is a non-exhausting number of known methods disclosed that often are employed for removing the undesired wax and hydrate formation, or preventing the formation of such:                Adding chemicals (such as antiscaling corrosion inhibitors, wax inhibitors, methanol or mono-ethylene glycol; MEG) to the well fluids.        Using direct electric heating (DEH), i.e. arranging electrical cables along the pipeline in order to maintain the well fluids at a temperature above the temperature at which wax precipitates (“wax appearance temperature”—WAT). A typical arrangement is an extra cable placed piggy back or besides the production pipe with an effect of several megawatt (typically 30-40 MW).        Thermal insulation in the form of applying thermal (insulation) around the pipeline and/or burying it in the seabed. Alternatively a pipe-in-pipe configuration or thick layer of PP (solid polypropylene foam) or PP (foam) or PUR (polyurethane) extruded around the pipes outer surface.        Rock dumping and dredging of pipelines, mainly performed to insulate the pipes further, thus keeping the flow warm.        Using a pig, as described above.        
Several disadvantages have been associated the use of pigs. A pigging system typically comprises a pig launching station and a retrieving station, each comprising an assembly of isolation valves, a trap barrel, an entry hatch and a bypass valve, enabling an operator to launch a pig (by support of a vessel) into the pipeline safely and to retrieve it at the other end. The trap barrels are generally closed at one end and situated outside the main pipeline. The system tends to take up a large volume and is heavy. Pigs are also launched from a top side facility (platform or land) and two costly production pipes are thus needed (pig loop) instead of just one. Also, the well stream production must in many cases be reduced in order not to impose too high pressure on the pig. As mentioned above it has recently been published a bi-directional pig apparatus that overcome these shortcomings. This pig apparatus disclosed in WO 2012/093079 A2 comprises a pig arranged for movement inside a pipe and includes a tubular body having a longitudinal axis coinciding with the central axis of the pipe and a through-going opening allowing fluids in the pipe to flow through the body. The pig apparatus also comprises propulsion means arranged and configured for imparting a motive force to the pig to allow the pig to move inside the pipe portion independently of the fluid flow. However, this system still necessitates a movable pig inside the pipe and propulsion means to impart the required pig movements, setting a complexity level that may prove costly and cumbersome. Even with the novel bi-directional plug operations based on plugging will face the risk of operational challenges such as pig obstruction due to excessive deposit formation.
With the exception of the pig apparatus disclosed in WO 2012/093079 A2 the measures taken to prevent formation or hydrate and wax deposits today have clear limits when it comes to transportation distance. The longer the pipe, the higher the cost.
A simple and reliable system for ensuring subsea transport of hydrocarbons over long distances is to allow so-called “cold flow”. If the well stream fluids, pipeline wall and the ambient seawater all are at the same or similar temperature, wax deposits do not form on the interior pipe wall surface, but are transported together with the well fluid without problems. Cold flow is normally achieved by allowing the well stream to be cooled to ambient seawater temperature simply by heat exchange through the pipeline wall. However, severe hydrate and wax formation will take place in the pipeline section where cooling takes place. This relatively short cooling section, typically 1000 m or less, will therefore have to go through deposit removing operations on a more frequently basis, for example by using the above mentioned pigs as disclosed in WO 2012/093079 A2 or by statically exposing the relevant section to heating for short period of time as disclosed in WO 2009/051495.
It is therefore a need for an apparatus and a method that removes deposits, particularly on the inside walls of pipe sections, in areas with excessive amount of deposits, for example around the cooling sections in case of the “cold-flow” process, and which overcome shortcomings of the prior art and obtains further advantages.